Hydraulic backstay tensioner

ABSTRACT

A self-contained tension adjusting device for adjusting the tension in the backstay of a sailboat includes a single housing for a pump, a hydraulic cylinder with a movable piston coupled to the backstay, and a reservoir containing hydraulic fluid. Operation of the pump forces hydraulic fluid from the reservoir to the cylinder to move the piston and adjust tension in the backstay. A bleed valve in the housing is operated to relieve tension in the backstay, and a relief valve in the housing limits the maximum tension in the backstay.

May 1, 1973 l1 ll [5 1 HYDRAULICBACKSTAYTENSHONER 3,620,182 11 1971 Russell..1...............,..............l14/109 Primary ExaminerEdgar W. Geoghegan Attorney-Christie, Parker & Hale ABSTRACT 22 Filed: May 13, 1971 A self-contained tension adjusting device for adjusting the tension in the backstay of a sailboat includes a single housing for a pump, a hydraulic cylinder with a movable piston coupled to the backstay, and a reser- "'i 9/04 voir containing hydraulic fluid. Operation of the pump Field of Search....................60/52 HA, 114/102, forces hydraulic fluid from the reservoir to the Appl.No.:

cylinder to move the piston and adjust tension in the backstay. A bleed valve in the housing is operated to [56] References Cited relieve tension in the backstay, and a relief valve in the housing limits the maximum tension in the backstay.

Van Cleave et .60/52 HA X 9 Claims, 2 Drawing Figures Barosko.H.1..............11......1.60/52 HA Ill HYDRAULIC BACKSTAY TENSIONER BACKGROUND OF THE INVENTION This invention relates to sailboats, and more particularly to means for adjusting the tension in the backstay on a sailboat.

The mast of a sailboat is supported by a headstay which slants toward the fore portion of the boat, and a backstay" which slants toward the aft portion of the boat. The backstay is adjusted to vary the tension in the headstay and thereby control the amount of sag in the rigging when the sail is under load, such as when the sailboat is going to weather. Increased tension in the stays increases the compression load in the mast.

It is often necessary to increase backstay tension during sailing to take up sag in the headstay. The load on the backstay can increase substantially over its static setting when the boat is exposed to gusts or heavy wind. Therefore, it is often necessary to reduce backstay tension immediately to limit compression loads in the mast and thereby prevent damaging the mast or hull.

In the past, backstay tension has been adjusted by a turnbuckle connected to the lower end of the backstay. A disadvantage of using a turnbuckle is that the skipper has no knowledge of the actual load on the mast and stay system during various sailing conditions. This deficiency can be especially serious if the load on the stay system increases suddenly, such as when the boat sails on the wind in a strong breeze or heavy gusts. The turnbuckle does not automatically relieve the sudden loading on the rigging, and it is difficult to make adjustments quickly on a turnbuckle when the backstay is under sailing-type loads.

Various hydraulic tensioning devices have been used previously to adjust backstay tension. Generally, these devices include a hydraulic reservoir and pump below decks, and hydraulic lines routed through the deck to a power cylinder connected to the backstay. So far as is known, no load limit feature is included in the prior art tensioning systems.

SUMMARY OF THE INVENTION This invention provides a hydraulic backstay tension adjusting device which provides an easy means for increasing or decreasing backstay tension during all sailing conditions. The invention is based on the recognition that prior art hydraulic tensioning devices are so costly and time consuming to install that their advantages do not significantly outweigh the disadvantages of the conventional turnbuckle.

Briefly, the tension adjusting device is entirely selfcontained in a single housing. A pumping element in the housing is actuated to force hydraulic fluid from a reservoir in one part of the housing to a hydraulic cylinder in another part of the housing. A movable piston in the cylinder is connected to the backstay, and hydraulic fluid forced into and released from the cylinder moves the piston to adjust the tension in the backstay.

The hydraulic tension adjuster of this invention provides substantial advantages over prior art hydraulic tensioning systems in which the pump, reservoir, selector valves, hydraulic lines, and the like are located remotely from the power cylinder. In the latter arrangement, it is costly and time consuming to install the reservoir and pump, and route hydraulic lines through the deck. Moreover, necessary space is taken up by the pump and reservoir, and there is chance of snagging a sail or line on parts leading away from the cylinder.

The present hydraulic tension adjuster, on the other hand, may be installed simply by coupling one end to the backstay and the other end to the backstay deck fitting, replacing the conventional turnbuckle. Moreover, the present self-contained unit routes most of the high pressure leakage paths back to the reservoir. This minimizes loss of fluid, and prevents hydraulic fluid from spilling on the deck, which sometimes occurs when the reservoir is located remote from the power cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawing, in which:

FIG. 1 is an elevation view showing a self-contained hydraulic tension adjusting device coupled to the backstay of a sailboat; and

FIG. 2 is a sectional elevation of the hydraulic tension adjusting device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a sailboat 10 includes an upright mast 12 supported by a headstay 14 which extends toward the fore portion of the boat, and a backstay 16 which extends toward the aft portion of the boat. A conventional turnbuckle 18 at the lower end of the headstay adjusts the position of the masthead. A selfcontained hydraulic tension adjusting device 20 coupled to the lower end of the backstay adjusts tension in both the headstay and the backstay.

Operation of hydraulic tension adjusting device 20 will be better understood by referring to FIG. 2, in which tension adjusting device 20 is shown as a selfcontained unit in a housing 22 having a hollow interior. Housing 22 generally includes an upwardly opening cup-like lower portion or base casting 24 which carries an O-ring seal 26 at its top sealed to an upright cylindrical, sleeve-like intermediate portion 28. A cap 30 is sealed to the top of intermediate portion 28 by an O- ring seal 31. A pair of upright, elongated bolts 32 outside of housing 22 are threaded into cap 30 and tightened against an outwardly extending shoulder 33 on lower portion 24 of the housing to rigidly secure the cap over the top of the housing. Alternatively, bolts 32 may be inside housing 22, with their lower ends threaded into housing lower portion 24.

A plurality of radially spaced apart vertical webs 34, which are integral with the inner wall of housing lower portion 24, extend inwardly toward the interior of lower portion 24. A seat 36 formed at the top of the webs 34 supports an upright, open-ended cylinder 38 which is centered in intermediate portion 28 of the housing. A least three of the webs 34 are preferred to properly locate and center cylinder 38. The preferred form of the invention includes five webs for supporting the cylinder. The top of cylinder 38 is sealed to cap 30 by an O-ring seal 40. Thus, an annular chamber 42 is formed between the exterior of cylinder 38 and the interior of intermediate portion 28 of the housing. The annular chamber forms the upper portion of the reservoir volume and expansion volume above the fluid level of hydraulic fluid (described in detail below) in the housing.

A piston 44 makes a sliding and sealing fit in cylinder 38. An elongated upwardly extending piston rod 46 extends from the piston out through the top of cap 30 for connection to an extension arm 48 connected to backstay 16. The lower end of housing 22 carries a bracket 49 which is adapted for fastening to the deck, bulkhead, hull, or the like of the sailboat. Thus, longitudinal movement of the piston in cylinder 38 will adjust the tension on the backstay. Cap 30 carries a pair of rings 50 which seal against piston rod 46 and prevent leakage of hydraulic fluid to the exterior of housing 22.

Piston 44 is sealed to the exterior of cylinder 36 by an O-ring seal 52. This divides the interior of the housing into a chamber 54 in the upper portion of cylinder 38, and a reservoir 56in annular chamber 42 and lower portion 24 of the housing. Chamber 54 contains hydraulic fluid 58 which is maintained at a high pressure in a manner described in detail below. Reservoir 56 contains hydraulic fluid 60 which is maintained at a lower pressure, generally atmospheric pressure.

A pump housing 62 secured to the bottom of housing lower portion 24 has a horizontally disposed, outwardly opening passage 64. A check valve 66 in passage 64 includes an elongated piston 660 which makes a sliding and sealing fit in the passage. An outwardly extending portion of the piston is connected to the lower end of an upright elongated handle 70 which, in turn, is pivotally mounted on a bracket 72 at the lower portion of housing 22. Pumping of handle 70 moves check valve piston 66a longitudinally back and forth in passage 64.

Check valve 66 further includes a movable ball 66b which has a clearance fit in a passage 66c ending in a valve seat 66d at the junction of a passage 66e of smaller diameter than passage 66c. Several radially extending passages 66f open through the body of the valve into passage 66:: for a purpose described in detail below.

An upright, elongated hydraulic line 74 extends from passage 64 up through the interior of lower portion 24 of housing 22. The upper end of line 74 is connected through a check valve 76 to an upright, elongated hydraulic line 78 which extends up through annular chamber 42. The upper end of line 78 opens into the upper portion of chamber 54 through a passage 80 formed in cap 30.

An upright passage 82 which extends through the bottom wall of lower portion 24 of the housing delivers low pressure hydraulic fluid 60 from reservoir 56 to passage 64 of pump housing 62.

In use, pumping of handle 70 slides check valve 66 (shown in a closed position in FIG. 2) to the left in FIG. 2 to force hydraulic fluid 58 out of the pump housing. Movement of the check valve to the left in FIG. 2 forces ball 66b back against valve seat 66d and traps the fluid in line 74 and forces it through check valve 76, hydraulic line 78, and passage 80 into chamber 54. The fluid under pressure in chamber 54 moves piston 44 downwardly in chamber 54 and thereby increases the tension on backstay 16.

On the reverse stroke of the check valve, i.e., moving to the right in FIG. 2, a partial vacuum is produced in line 74 which causes ball 66b to open and allow fluid from reservoir 56 to flow through passage 82, through passages 66f, and past the ball 66b. A cross pin 66g or a wire across the opening of passage 66c prevents the ball from dropping into passage 64 during the return stroke.

A tension gauge 83 is mounted in a port 84 which opens through cap 30 and communicates with high pressure hydraulic fluid 58 in line 78 and passage 80. The gauge senses the pressure exerted on piston 44 by the high pressure fluid in chamber 54. The gauge is calibrated to produce a scale reading of the load, in pounds, exerted by the piston on the backstay. Thus, the scale on the gauge provides the skipper with an immediate indication of the tension on the backstay so that the desired loading on the backstay may be adjusted, if necessary.

A horizontally disposed, adjustable bleed valve 86 makes a sliding and sealing fit in a passage 88 extending through cap 30 to communicate with the high pressure hydraulic fluid in port 84, passage 80, and chamber 54. An adjusting screw 89 adjusts the position of the bleed valve ball in passage 88. A return passage 90 in cap 3 1') extends down from passage 88 to open into the upper portion of annular chamber 38 above reservoir 56. The bleed valve ball is normally closed (as shown in FIG. 2) to block the flow of high pressure fluid from the right side (as viewed in FIG. 2) of the ball through return passage 90.

In use, bleed valve 86 may be adjusted to relieve the tension on the backstay 16. A typical instance where it would be desirable to relieve the load on the backstay would be when the boat is to be left at the dock. Other changes in backstay tension might be made while sailing upon the discretion of the skipper.

To relieve backstay tension, adjusting screw 89 is turned to move it to the left (as viewed in FIG. 2) in passage 88 to move bleed valve 86 to the left and permit a portion of the high pressure fluid in passage 84 to flow through return passage 90 into annular chamber 42 above reservoir 56. The return rate of flow to the reservoir is increased by moving the bleed valve farther to the left in passage 86. Return flow of the hydraulic fluid to the reservoir reduces pressure on the piston 44 and permits it to move up in chamber 54 to reduce the tension in backstay 16. i

A filler port 92 extends vertically down through cap 30 and opens into the upper portion of annular chamber 42 on the side of the housing opposite bleed valve 86. Hydraulic fluid may be added to the reservoir through the filler port. A cap screw 94 is threaded into the port to close it.

A relief valve 96 in the filler port is adjustable to limit the load exerted on the backstay to a predetermined maximum amount established by the particular setting of the relief valve. Relief valve 96 includes a cylindrical body 98 the upper portion of which is sealed in passage 92 by an O-ring seal 100. The upper portion of the body has a lateral passage 101 which opens into a stepped bore formed by a narrow vertical passage 102 in the upper portion of the body which, in turn, opens into an elongated downwardly opening, larger diameter vertical passage 104 in the lower portion of the body. An upwardly and outwardly extending passage 106 in cap 30 delivers high pressure fluid from chamber 54 to lateral passage 101 of the relief valve.

Relief valve 96 is normally closed (as shown in FIG. 2) by a ball 108 which is seated on a seat 110 at the bottom of narrow vertical passage 102, thereby preventing fluid in chamber 54 from flowing through the interior of the valve and into annular chamber 42 above reservoir 56. The ball is urged upwardly against seat 1 by a spool 112 supported by a coil spring 114. The load exerted on the ball by the spool is adjusted by a head 116 threaded into the lower portion of passage 104.

In use, the desired maximum load for backstay 16 is adjusted by removing relief valve 96 from passage 92 and turning head 116 to move it up or down in passage 104 by means of a tool, such as a screwdriver, to properly adjust the load exerted by spring 114 on ball 108. This adjustment, of course, regulates the pressure required to move the ball from its seat. Thus, when the relief valve is placed in passage 92 in the position shown in FIG. 2, ball 108 senses fluid pressure in chamber 54. When pressure in the chamber exceeds the load imposed on the ball by spring 114, the ball moves from its seat and provides a return flow path for fluid through passage 106 to the interior of the relief valve. Holes (not shown) extending transversely through body 98 allow the relief flow of fluid to return to reservoir 56.

The relief valve may be set to open at a desired maximum loading on backstay 16. This adjustment is made by pumping fluid into chamber 54 and observing load gauge 83 to determine the maximum load exerted on the backstay just prior to when the relief valve is opened. If the maximum load indicated on the gauge is different from the desired amount, the relief valve is removed and head 116 is adjusted so that the tension in spring 114 is adjusted accordingly. The valve is replaced, fluid is pumped into chamber 54 again, and the above steps are repeated until the desired maximum load is established. Thereafter, when the load on the backstay reaches the pre-established maximum level, the relief valve senses the load and immediately opens to permit high pressure fluid from chamber 54 to flow back into reservoir 56 so that the load exerted on the backstay does not exceed the pre-established maximum level.

A typical instance where the relief valve is useful would be when the boat encounters strong winds and the load on the backstay increases substantially over the static load. For example, many sailboats recently have been built with relatively tall riggings, and in heavy winds the loading on the rigging can increase as much as 200 percent over the static load. In these cases, it often becomes necessary to limit tension in the backstay to prevent damaging the mast or hull of the boat.

Iclaim:

1. In a sailboat having a mast supported by backstay, apparatus for adjusting tension in the backstay including a housing having mans for attachment to the sailboat; a reservoir in the housing containing hydraulic fluid; a cylinder in the housing adapted to receive hydraulic fluid under pressure from the reservoir; a piston movable in the cylinder in response to hydraulic fluid being admitted to the cylinder; a rod attached to the piston and extending outwardly from the housing to provide means for connection to the backstay, the piston and cylinder cooperating to define a high pressure chamber arranged such that hydraulic fluid being admitted under pressure from the reservoir retracts the piston into the housing and shortens the effective length of the piston rod; means for delivering hydraulic fluid under pressure from the reservoir to the high pressure chamber to retract the piston and shorten the effective length of the piston rod to increase the tension in the backstay; a hydraulic line providing a flow path for hydraulic fluid from the high pressure chamber to the reservoir; and a normally closed relief valve in the hydraulic line for sensing hydraulic pressure existing in the high pressure chamber, the relief valve being adjustable to automatically open when the pressure sensed by it exceeds a preselected value to channel hydraulic fluid from the high pressure chamber back to the reservoir to allow for movement of the piston rod responsive to excessive tension in the backstay to thereby limit the tension in the backstay to said preselected value.

2. Tension adjusting apparatus according to claim 1 including a second hydraulic line for delivering the hydraulic fluid to the high pressure chamber from the reservoir, pumping means movable in the second hydraulic line for forcing fluid from the reservoir to the high pressure chamber, and a check valve in the second hydraulic line downstream of the pumping means for maintaining pressure on the hydraulic fluid pumped to the high pressure chamber from the reservoir.

3. Tension adjusting apparatus according to claim 1 including a second hydraulic line in the housing providing a flow path for hydraulic fluid from the high pressure chamber to the reservoir, and a bleed valve in the housing and movable in the second hydraulic line for controlling the flow of hydraulic fluid into the reservoir, whereby tension in the backstay may be reduced by moving the bleed valve to channel hydraulic fluid from the high pressure chamber into the reservoir to extend the piston and increase the effective length of the piston rod.

4. Tension adjusting apparatus according to claim 2 including means in communication with the second hydraulic line and located downstream of the check valve for measuring the pressure exerted on the piston by the hydraulic fluid in the high pressure chamber, and means for indicating said pressure.

5. In a sailboat having a mast supported by a backstay, a self-contained hydraulic tension adjusting device connected to the backstay comprising a housing; a reservoir in the housing containing hydraulic fluid; a cylinder in the housing; an elongated movable piston in the cylinder, a portion of the piston extending out of the housing and connected to the backstay; a first hydraulic line in the housing extending from the reservoir to the cylinder to deliver hydraulic fluid between the reservoir and the cylinder; a pumping element in the housing and movable in the first hydraulic line for forcing hydraulic fluid from the reservoir to the cylinder to move the piston and increase the tension in the backstay; a second hydraulic line in the housing providing a flow path for hydraulic fluid from the cylinder to the reservoir; a selectively operated bleed valve in the housing and movable in the second hydraulic line from a closed position blocking the flow of hydraulic fluid into the second hydraulic line to an open position for controlling the flow of hydraulic fluid into the reservoir, whereby tension in the backstay may be reduced by moving the bleed valve to open it and channel hydraulic fluid into the reservoir; a third hydraulic line in the housing providing a flow path for hydraulic fluid from the cylinder to the reservoir; and a normally closed relief valve in the third hydraulic line for sensing the pressure exerted on the piston by the hydraulic fluid, the relief valve being adapted to open and channel hydraulic fluid back to the reservoir when the pressure sensed exceeds a predetermined limit.

6. A hydraulic tension adjusting device according to claim including means in the housing for measuring the pressure exerted on the piston b the hydraulic fluid, and means on the housing for providing an indication of the measured pressure.

7. Apparatus for adjusting the tension in a backstay of a sailboat having a mast supported by the backstay including a housing adapted for attachment to the sailboat; a reservoir in the housing containing hydraulic fluid; a cylinder in the housing adapted to receive hydraulic fluid under pressure from the reservoir; a

' piston movable. in the cylinder in response to hydraulic fluid being admitted to the cylinder; a rod attached to the piston and extending outwardly from the housing to provide means for connection to the backstay, the piston and cylinder cooperating to define a high pressure chamber arranged such that hydraulic fluid being admitted under pressure from the reservoir tends to retract the piston into the housing and shorten the effective length of the piston rod; means for delivering hydraulic fluid under pressure from the reservoir to the high pressure chamber to retract the piston and shorten the effective length of the piston rod to adjust the tension in the backstay; a hydraulic line providing a flow path for hydraulic fluid from the high pressure chamber to the reservoir; and a normally closed relief valve in the hydraulic line for sensing the hydraulic pressure existing in the high pressure chamber, the relief valve being adjustable to automatically open when the pressure sensed by it exceeds a preselected value to channel hydraulic fluid from the high pressure chamber back to the reservoir to allow for movement of the piston rod responsive to excessive tension in the backstay to thereby limit the tension in the backstay to said preselected value.

8. Tension adjusting apparatus according to claim 7 including a second hydraulic line for delivering the hydraulic fluid to the high pressure chamber from the reservoir, pumping means movable in the second hydraulic line from forcing fluid from the reservoir to the high pressure chamber, and a check valve in the second hydraulic line downstream of the pumping means for maintaining pressure on the hydraulic fluid admitted to the high pressure chamber from the reservoir.

9. Tension adjusting apparatus according to claim 8 including means in communication with the second hydraulic line and located downstream of the check valve for measuring the pressure exerted on the piston by the hydraulic fluid in the high pressure chamber, and means for indicating said pressure. 

1. In a sailboat having a mast supported by backstay, apparatus for adjusting tension in the backstay including a housing having mans for attachment to the sailboat; a reservoir in the housing containing hydraulic fluid; a cylinder in the housing adapted to receive hydraulic fluid under pressure from the reservoir; a piston movable in the cylinder in response to hydraulic fluid being admitted to the cylinder; a rod attached to the piston and extending outwardly from the housing to provide means for connection to the backstay, the piston and cylinder cooperating to define a high pressure chamber arranged such that hydraulic fluid being admitted under pressure from the reservoir retracts the piston into the housing and shortens the effective length of the piston rod; means for delivering hydraulic fluid under pressure from the reservoir to the high pressure chamber to retract the piston and shorten the effective length of the piston rod to increase the tension in the backstay; a hydraulic line providing a flow path for hydraulic fluid from the high pressure chamber to the reservoir; and a normally closed relief valve in the hydraulic line for sensing hydraulic pressure existing in the high pressure chamber, the relief valve being adjustable to automatically open when the pressure sensed by it exceeds a preselected value to channel hydraulic fluid from the high pressure chamber back to the reservoir to allow for movement of the piston rod responsive to excessive tension in the backstay to thereby limit the tension in the backstay to said preselected value.
 2. Tension adjusting apparatus according to claim 1 including a second hydraulic line for delivering the hydraulic fluid to the high pressure chamber from the reservoir, pumping means movable in the second hydraulic line for forcing fluid from the reservoir to the high pressure chamber, and a check valve in the second hydraulic line downstream of the pumping means for maintaining pressure on the hydraulic fluid pumped to the high pressure chamber from the reservoir.
 3. Tension adjusting apparatus according to claim 1 including a second hydraulic line in the housing providing a flow path for hydraulic fluid from the high pressure chamber to the reservoir, and a bleed valve in the housing and movable in the second hydraulic line for controlling the flow of hydraulic fluid into the reservoir, whereby tension in the backstay may be reduced by moving the bleed valve to channel hydraulic fluid from the high pressure chamber into the reservoir to extend the piston and increase the effective length of the piston rod.
 4. Tension adjusting apparatus according to claim 2 including means in communication with the second hydraulic line and located downstream of the check valve for measuring the pressure exerted on the piston by the hydraulic fluid in the high pressure chamber, and means for indicating said pressure.
 5. In a sailboat having a mast supported by a backstay, a self-contained hydraulic tension adjusting device cOnnected to the backstay comprising a housing; a reservoir in the housing containing hydraulic fluid; a cylinder in the housing; an elongated movable piston in the cylinder, a portion of the piston extending out of the housing and connected to the backstay; a first hydraulic line in the housing extending from the reservoir to the cylinder to deliver hydraulic fluid between the reservoir and the cylinder; a pumping element in the housing and movable in the first hydraulic line for forcing hydraulic fluid from the reservoir to the cylinder to move the piston and increase the tension in the backstay; a second hydraulic line in the housing providing a flow path for hydraulic fluid from the cylinder to the reservoir; a selectively operated bleed valve in the housing and movable in the second hydraulic line from a closed position blocking the flow of hydraulic fluid into the second hydraulic line to an open position for controlling the flow of hydraulic fluid into the reservoir, whereby tension in the backstay may be reduced by moving the bleed valve to open it and channel hydraulic fluid into the reservoir; a third hydraulic line in the housing providing a flow path for hydraulic fluid from the cylinder to the reservoir; and a normally closed relief valve in the third hydraulic line for sensing the pressure exerted on the piston by the hydraulic fluid, the relief valve being adapted to open and channel hydraulic fluid back to the reservoir when the pressure sensed exceeds a predetermined limit.
 6. A hydraulic tension adjusting device according to claim 5 including means in the housing for measuring the pressure exerted on the piston b the hydraulic fluid, and means on the housing for providing an indication of the measured pressure.
 7. Apparatus for adjusting the tension in a backstay of a sailboat having a mast supported by the backstay including a housing adapted for attachment to the sailboat; a reservoir in the housing containing hydraulic fluid; a cylinder in the housing adapted to receive hydraulic fluid under pressure from the reservoir; a piston movable in the cylinder in response to hydraulic fluid being admitted to the cylinder; a rod attached to the piston and extending outwardly from the housing to provide means for connection to the backstay, the piston and cylinder cooperating to define a high pressure chamber arranged such that hydraulic fluid being admitted under pressure from the reservoir tends to retract the piston into the housing and shorten the effective length of the piston rod; means for delivering hydraulic fluid under pressure from the reservoir to the high pressure chamber to retract the piston and shorten the effective length of the piston rod to adjust the tension in the backstay; a hydraulic line providing a flow path for hydraulic fluid from the high pressure chamber to the reservoir; and a normally closed relief valve in the hydraulic line for sensing the hydraulic pressure existing in the high pressure chamber, the relief valve being adjustable to automatically open when the pressure sensed by it exceeds a preselected value to channel hydraulic fluid from the high pressure chamber back to the reservoir to allow for movement of the piston rod responsive to excessive tension in the backstay to thereby limit the tension in the backstay to said preselected value.
 8. Tension adjusting apparatus according to claim 7 including a second hydraulic line for delivering the hydraulic fluid to the high pressure chamber from the reservoir, pumping means movable in the second hydraulic line from forcing fluid from the reservoir to the high pressure chamber, and a check valve in the second hydraulic line downstream of the pumping means for maintaining pressure on the hydraulic fluid admitted to the high pressure chamber from the reservoir.
 9. Tension adjusting apparatus according to claim 8 including means in communication with the second hydraulic line and located downstream of the check valve for measuring the pressure exerted on the piston by the hYdraulic fluid in the high pressure chamber, and means for indicating said pressure. 